In the gasification of an ash-containing fuel, synthesis gas is prepared by partially combusting the fuel with an oxygen-containing gas. The fuel used for this purpose can be coal, but lignite, peat, wood and liquid fuels such as shale oil and oil from tar sands are also suitable. The oxygen-containing gas may be air, but oxygen-enriched air or pure oxygen can also be utilized.
Gasification is effected in a reactor. For preference the reactor has substantially the shape of a circular cylinder, arranged vertically. Other shapes such as a block, sphere or cone, however, are also possible. The operating pressure in the reactor is generally between 1 and 70 bar.
Besides the fuel and the oxygen-containing gas, a moderator is conveniently passed into the reactor as well. Said moderator exercises a moderating effect on the temperature of the gasification reaction by entering into an endothermic reaction with the reactants and/or the products. Suitable moderators are steam and carbon-dioxide.
The fuel, the oxygen-containing gas and the moderator are preferably passed into the reactor through at least one burner. The number of burners is advantageously at least two. In a suitable embodiment, the burners are arranged symmetrically in relation to the axis of the reactor, in a low-lying part of the reactor wall.
In the gasification reaction, slag is formed in addition to synthesis gas. A large proportion of the slag falls down and disappears from the reactor through the slag discharge. It has been found, however, that a proportion of the slag is entrained with the product gases to the discharge pipe. The entrained slag is in the form of small droplets or porous particles. It is called fly slag and can create severe disturbance by causing contamination in the equipment. Contamination takes place especially if the fly slag is glutinous, which is the case at a temperature where the slag is no longer entirely molten but not yet completely solidified either. That temperature is in a range that may cover several hundred degrees centigrade and is generally between 700 and 1500.degree. C. When the fly slag leaves the reactor it generally has a temperature of between 1000 and 1700.degree. C. In order to prevent contamination as far as possible, the discharged synthesis gas with the fly slag is quenched, so that the fly slag rapidly solidifies. Said quenching is preferably effectuated by injecting a cold gas and/or water into the gas discharge pipe. After the gas has cooled down the fly slag is removed from the gas, for example by means of one or more cyclones.
When the fly slag has been separated from the synthesis gas, all the fly slag is in the form of fine, porous particles. Said particles exhibit the property that the heavy metals contained therein can be lixiviated by water. Consequently they form a potential source of environmental pollution when said fine slag particles are stored outdoors. A proportion of the fuel in the fly slag is not converted into synthesis gas. The solidified fly slag therefore contains a considerable percentage of carbon.
The heavy metals are not lixiviated by water from the slag which is obtained through the slag discharge. That makes outdoor storage possible without any danger of environmental pollution. The slag obtained in this way can also be used for road construction. The carbon content of this slag is generally lower than 1% by weight.
It has been found that if the fly-slag is remelted, this yield slag from which heavy metals are not readily lixiviated.
It has been proposed to recycle the fly-slag particles via the burners to the reactor together with the fuel to be gasified, so that said particles are again contacted with oxygen. In this way practically all the carbon in the fly slag is partially combusted. Even more importantly, the fly slag then melts again and at least a proportion thereof falls down to the slag discharge. However, this proposal has the drawback that a proportion of the recycled fly-slag particles are again entrained with the synthesis gas.
That means that more fly slag has to be separated in the cyclones, so that the latter have to be larger and therefore more expensive. Moreover, the pneumatic transport of fly slag to the reactor requires a considerable quantity of carrier gas. These quantities may become such as to have an adverse effect on the thermal efficiency of the combustion and therefore the carbon monoxide and hydrogen yield.